The present invention relates to a semiconductor laser module, and particularly relates to a semiconductor laser module having a fiber grating (FG) (or embedded fiber grating reflector) structure which is formed from a diffraction grating incorporated in an optical fiber.
Heretofore, a method in which a fiber grating or embedded fiber grating reflector structure (sometimes abbreviated to "FG structure") is formed from a diffraction grating incorporated in an optical fiber receiving laser light emitted from a laser diode is often employed in a semiconductor laser module. By using the fiber grating, stabilization of the wavelength (for example, 0.98 .mu.m band) of emitted laser light from the module can be attained and this wavelength can be controlled (for example, ELECTRONIC LETTERS Vol. 30 No. 25 (1994. 12. 8), p. 2147-2149, LEOS Conf. Digest, New Semiconductor Laser Devices and Applications (1990)). The semiconductor laser module of this type is popularly used in optical fiber communication, or the like. For example, as a semiconductor laser module in which a single fiber grating is incorporated, it is known that in which a fixed pitch of refractive index variation grating (FG) is formed in a place distanced by 10 cm to the order of meters from the laser chip, that is, from an end surface of the optical fiber is known.
In the case where this fiber grating (FG) is present in the outside of the package in which the laser chip is installed, the optical fiber between the laser diode (LD) and the fiber grating (FG) may suffer from stress such as bending or stretching. In a resonator formed between the laser diode and the fiber grating, the state of light polarization changes easily because of the stress of the optical fiber. Accordingly, the output characteristics of the laser module fluctuates easily.
In the case of a light-emitting module in which a fiber grating is incorporated, it may be thought of to insert an optical fiber into a tubular structure called "ferrule" and incorporate a diffraction grating in the optical fiber in the ferrule so that the optical fiber is protected from such disturbance.
If the FG diffraction grating is incorporated in such a structure, however, the distance between the laser diode chip (strictly, the light emitting surface thereof) and the diffraction grating is shortened. Accordingly, the output characteristics of the module become unstable, because wavelength difference of the longitudinal mode, which is defined by the distance between the laser chip and FG, is large and the contribution of each mode is reflected more clearly to the output of the module. This results as kinks in the optical output power characteristics against the external conditions such as driving current and operating temperature.
Hence, it has been difficult to incorporate a fiber grating structure into a ferrule.